NASA Technical Reports Server (NTRS) 19770012472: Continuous plasma laser. [method and apparatus for producing intense, coherent, monochromatic light from low temperature plasma

United States Patent [19]

Fletcher et al.

[45]

4,007,430
Feb. 8, 1977

[54] CONTINUOUS PLASMA LASER

[76] Inventors: James C. Fletcher, Administrator of
the National Aeronautics and Space
Administration, with respect to an
invention of; Willard F. Libby, Los
Angeles, Calif.; Carl A. Jensen,
Davis, Calif.; Lowell L. Wood, Simi,
Calif.

[22] Filed: Aug. 10, 1971

[21] Appl. No.: 170,544

Related U.S. Application Data

[60] Division of Ser. No. 866,442, Oct. 14, 1969, Pat. No.

3,617,804, which is a continuation of Ser. No.

479,357, Aug. 12, 1965, abandoned.

[52] U.S. Cl 331/94.5 D; 33 1/94.5 G;

331/94.5 PE

[51] Int. Cl. 2 HOIS 3/00

[58] Field of Search 331/94.5 D, 94.5 G,

331/94.5 PE

[56] References Cited

UNITED STATES PATENTS

3,395,364 7/1968 Bridges 331/94.5

3,413,568 11/1968 Gordon et al 331/94.5

OTHER PUBLICATIONS

Handbook of Chemistry and Physics, The Chemical

Rubber Company, 44th Edition, 1962-1963, p. 3097.

Gordon et al: “Continuous Visible Laser Action in

Singly Ionized Argon, Krypton and Xenon,” Applied
Physics Letters vol. 4, May 15, 1964, pp. 178-180.

Primary Examiner — Edward S. Bauer
Attorney, Agent, or Firm — Monte F. Mott; Wilfred
Grifka; John R. Manning

[57] ABSTRACT

A method and apparatus for producing intense, coher-
ent, monochromatic light from a low temperature
plasma are disclosed. The apparatus includes a housing
for confining a gas at subatmospheric pressure and
including a set of reflectors defining an optical cavity.
At least one anode and cathode are positioned within
the gas. First control means control the voltage applied
to the anode and second control means independently
control the temperature of the cathode. The pressure
of the gas is controlled by a third control means. An
intense monochromatic output is achieved by confining
the gas in the housing at a controlled pre-determined
reduced pressure, independently controlling the tem-
perature of the electron emitting cathode and applying
a predetermined controlled low voltage to the anode.
An intermediate mode current is drawn from the cath-
ode and produces in the confined gas, a region having
a high density of metastable atomic states leading to a
population inversion. A low temperature, high-density
plasma is continuously produced in said region leading
to laser emission between said reflectors. Intense, co-
herent monochromatic light is emitted as a result of the
recombination of ions and electrons in the plasma.

11 Claims, 5 Drawing Figures

4,007,430

CONTINUOUS PLASMA LASER
ORIGIN OF THE INVENTION

The invention described herein was made in the per-
formance of work under a NASA contract and is sub-
ject to the provisions of Section 305 of the National
Aeronautics and Space Act of 1958 , Public Law
85-568 (72 Stat. 435 ; 42 USC 2457 ).

CROSS-REFERENCE TO RELATED
APPLICATIONS

The present application is a divisional of Ser. No.
866,442, filed Oct. 14, 1969, (now U.S. Pat. No.
3,617,804), which in turn is a stream-lined continua-
tion of Ser. No. 479,357, filed Aug. 12, 1965 and now
abandoned.

BACKGROUND OF THE INVENTION

1 . Field of the invention

This invention relates to a method and apparatus for
producing both noncoherent and coherent intense
monochromatic light from a continuous low tempera-
ture plasma. More particularly, this invention relates to
a method and apparatus for the production of radiation
in the form of intense monochromatic light from a
continuous low temperature plasma generated in a
region having a high density of atoms in metastable
atomic states and in which the neutralization of ions by
recombination with electrons proceeds at a very rapid
rate. By selecting the gas it is possible to produce light
at many different wavelengths. The use of helium to
give 584-A ionizing light is an example of an interesting
case. The method and apparatus are applicable both to
noncoherent light sources and to coherent light sources
of the commonly called lasers.

2. Description of the Prior Art

As has been set forth bv two of the inventors in an
article entitled, “Intense 584-A Light From a Simple
Continuous Helium Plasma,” which was originally pub-
lished in Volume 135, No. 5-A of “The Physical Re-
view” at pages A-1247 through A-1252 on Aug. 31,
1964, the importance of ionizing radiation in inducing
chemical reactions is widely recognized. An intense
monochromatic source of light in the far ultraviolet
region is highly desirable in space research to study the
chemical effects of solar ionizing ultraviolet radiation.
Such a source, of course, also has many other scientific
and industrial applications which are well known to
those skilled in the art. It should be noted, for example,
that the particular source described herein is also suit-
able for use as a vacuum ultraviolet laser when hydro-
gen or helium three is used in combination with helium
four as will become apparent from the discussion be-
low. Of course, the more common laser wavelengths
may also be induced by an appropriate selection of
gases and conditions, i.e., temperature, pressure, cur-
rent, voltage, and magnetic field intensity.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a
method and apparatus for producing intense mono-
chromatic light from a continuous plasma source.

It is a more particular object of this invention to
provide a method and apparatus for producing a mono-
chromatic source of ultraviolet light of very short wave-
lengths having an intensity per unit of bandwidth which
is greater than has heretofore been available. Depend-

ing on the environment this light may or may not be
coherent, such as “laser” light is.

Briefly stated in very general terms these objects are
achieved by producing, in a continuous plasma formed
5 by a gas confined at reduced pressure, a region having
a high density of atoms in metastable atomic states
and/or as ions. Upon recombination of the ions and
electrons in the plasma high energies are relased in the
form of intense radiations. The ions are usually formed
10 as result of a high density of metastable states. The
metastable states and/or the ions may be made to form
population inversions which may lead to the produc-
tion of coherent electromagnetic radiation.

In “The International Dictionary of Physics and Elec-
15 tronics” published by D. Van Nostrand Company, New
York, 1961, at page 741 there is given a discussion of
the “metastable state” which defines the sense in which
the term is used herein. Broadly speaking, this state is
one in which a system has acquired energy beyond that
20 for its most stable state yet has become relatively stable
in this high energy condition. Familiar daily examples
of this phenomenon are the fact that water at atmo-
spheric pressure may be heated several degrees above
its normal boiling point and yet not boil until the system
25 is disturbed from some external source. In the metasta-
ble state the water has received energy beyond that
normally required for liquid-vapor equilibrium and yet
has not become unstable. The system will, however,
flash into steam when it is disturbed. This term has
30 similarly been used in atomic and nuclear physics to
designate excited atomic states from which all possible
quantum transitions to lower states are partially or
completely forbidden transitions by the appropriate
selection rules. The method and apparatus for produc-
35 ing a plasma having a region of high density of metasta-
ble atomic states and/or ions (which may be considered
as a variety of a metastable state) to generate an in-
tense ultraviolet or other light will be described in de-
tail below.

40 In the drawings, wherein like reference characters
refer to like parts throughout:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side elevation of an em-
45 bodiment of the laser light source apparatus according
to the invention;

FIG. 2 is a sectional end elevation on the line 2—2 of
FIG. 1;

FIG. 3 is an enlarged cross-sectional view of the cath-
50 ode supporting conduit shown in FIG. 1;

FIG. 4 is a view similar to FIG. 3 but showing the
anode supporting conduit; and

FIG. 5 is a view showing a laser of the type shown in
FIG. 1 also including magnetic field generating means.

55 DESCRIPTION OF THE PREFERRED

EMBODIMENTS

The laser light source according to the invention
comprises in combination: housing means to confine a
60 gas at a pressure below atmosphere; at least one anode
and one cathode positioned in said gas in electrical
circuit relationship with each other; first control means
to control the voltage applied to the anode; second
control means to control the temperature of the cath-
65 ode by regulating power supply to it; third control
means to control the pressure of said gas; said first,
second and third control means being arranged to func-
tion independently of each other to permit independent

4,007,430

3

4

control of the temperature and density of the plasma metastable atomic states occur abundantly. In this re-
formed in said gas when an intermediate mode current gion there is rapid production of an recombination of

is drawn from said cathode to said anode and optical ions and electrons and each recombination in turn

means to establish a preferred light direction for stimu-
lated emission of radiation to obtain laser action. The
laser apparatus may also include means to cool the
anode and cathode support and magnetic field genera-
tion means to produce a magentic field along the pre-
ferred light direction.

As an illustration of the plasma producing system of
the invention, the hot cathode can be manufactured
from a 3-cm long, 0.0 10-inch diameter thoriated tung-
sten wire positioned parallel to and 1-cm from a 3-cm
long, 0.020 inch diameter tantalum wire serving as the
anode. For helium gas the pressure should be main-
tained between 30 and 1,000 microns typically 300
microns. When the anode voltage is controlled between
20 and 100 volts, and the current from the cathode is
more than 0.10 ampere but less than 100 amperes, an
intermediate mode current is drawn. Monochromatic
light in the far ultraviolet at 584-A is emitted, when 30
volts were applied to the anode and one ampere of
current flowed between the anode and cathode at 300
microns pressure.

The device has three current modes depending on
cathode temperature, anode voltage and helium pres-
sure. In the low current mode the device operates as a
vacuum diode tube. The intermediate current mode of
a hundred or more milliamperes to a few tens of am-
peres is the condition in which the device operates as
desired and as described herein. In the high current
mode the device shows negative resistance and oper-
ates as a low pressure arc discharge device in which a
current of more than 100 amperes is drawn. In the
intermediate current mode the internal resistance of
the device is positive and affords stable operations
without the use of an external resistance. In the high
current mode an external resistance is needed for sta-
ble operation.

The desired intermediate-current mode of operation
of the device is defined not only by the above-noted
current flow of more than 100 milliamperes but less
than 100 amperes, but also by the fact that this current
mode is achieved by maintaining the tungsten cathode
at on the order of 2500° K or more, by applying an
anode voltage in the range of 20 to 100 volts for helium
gas at a pressure of approximately 300 microns and the
electrode spacing, type, and geometry described here.
If the anode voltage exceeds 100 volts the device goes
into the high current mode under the conditions de-
scribed here. The onset of the high current mode will
depend on the nature of the use, the system geometry,
and other system parameters such as the pressure.
More generally the intermediate current mode is that
set of operating conditions under which the device
carries many times the space charge limited current of
a similar device operating in a vacuum, but the resis-
tance of the device is still positive. The high current or
arc state, on the other hand, is defined as the negative
resistance, region of operation of the device.

The unique emission characteristics of the device in
the intermediate current mode state are believed to be
related to the fact that it is operated with a very hot
cathode which provides abundant electrons and hence
high currents at relatively low anode voltages. There is
thus provided an abundant supply of low energy elec-
trons. This in turn results in a dense low temperature
plasma and an excited atomic state region in which

eventually produces a photon. Thus, the photon flux
5 density from transition going to the ground state of the
principal gas under conditions which do not radically
shift or broaden the light emitted by the atoms excited
directly or via the metastable states, but not the ions, is
proportional to the recombination rate which is high in
10 the present high density plasma arrangement. On the
other hand, the line width or bandwidth of the emitted
radiation is an exponential function of the temperature.
Hence, the low temperature plasma produced under
the above-noted conditions will result in a monochro-
15 matic emission of high intensity, which is able to escape
complete resonance trapping in its own parent gas.

As will be explained in detail below, the region
around the electrodes has been observed to have a high
density of metastable atomic states which, of course,
20 are predecessors of the ions. Such an inverted popula-
tion which holds here for the ions as well as for the
metastable states is known to afford the basis for laser
action. The device may be used with natural gases such
as helium, neon, krypton, xenon, hydrogen, and iso-
25 topes of these gases. Gases other than natural helium
have been used both alone and by injecting them into
the chamber in the presence of or in combination with
helium. When, for example, hydrogen at a pressure
which is a small fraction of that of the helium, is in-
jected in the presence of the inverted helium popula-
tion, an inverted population of hydrogen states occurs.
This results in the emission of light characteristic of
hydrogen, notably the 1216- A Lyman alpha line in the
35 vacuum ultraviolet. When an arrangement, such as
mirrors in an optical cavity, is provided, which provides
or tends to provide a preferred direction for the stimu-
lation of radiation, “laser” action results and coherent
light will be produced. Such arrangements are shown in
40 FIGS. 1 and 2 which will be described in detail below.

It has been found that substantially the same operat-
ing conditions will optimize the light intensity and the
number of metastable states. In previously known
plasma generating devices only two variables have been
45 available for control to produce inverted populations:

( 1 ) the gas pressure and (2) the electric field strength.
In the present device in in addition to controlling these
two factors, the heated cathode also affords control
over the cathode temperature to permit the generation
50 of low energy electrons. The current and the plasma
temperature in the device of the present invention are
thus necessarily a function of the cathode temperature,
the field strength and the pressure rather than simply
the field strength and pressure alone.

55 Thus, in the above described particular exemplary
embodiment it was found that if the pressure drops to
less than 50 microns or if the cathode temperature is
too low to cause significant emission, or if the anode
voltage falls to less than about 22 volts, the device
60 drops into the low current mode. The transition is grad-
ual in the case of varying cathode temperature, but
very abrupt with variation in pressure or anode voltage.
A drop of less than one-tenth of a volt on the anode will
cause the current to change from many amperes to a
65 few milliamperes. With very pure helium and well-out-
gassed electrodes the process is reversible at the same
voltage; otherwise there is a hysterisis effect and higher
starting voltage is necessary, although again the current

5

4 ,

,430

6

goes from milliamperes to amperes in about one-tenth
volt.

Within the intermediate current mode range, anode
current is determined primarily by cathode tempera-
ture. Anode voltage has little effect once it has been
raised past the striking level. Separation between cath-
ode and anode has been increased up to 7-cm with no
appreciable increase in anode voltage required. The
limiting factor on the current is the amount of heat the
anode can dissipate before it melts, or the heat the lead
in conductors can dissipate.

In the intermediate current mode the source resis-
tance is positive and therefore no resistance is required
in series with it to give stable operation. With a very
stable low output impedance power supply there was
no evidence of oscillation up to frequencies of 100
megacycles per second. The high current mode occurs
when anode potentials are increased to several hundred
volts. This is a typical low pressure arc and a resistor
must be placed in series with the source to obtain stable
operation.

The intensity of 584- A light is greatest from the cath-
ode region. The helium 584-A light intensity is also
roughly proportional to current flow and inversely pro-
portional to anode voltage. The maximum intensity is
obtained at around 300 microns of helium pressure for
the particular configuration described with respect to
the exemplary embodiment. The 121 6-A hydrogen
Lyman alpha line is emitted when hydrogen gas is intro-
duced into the system. The intensity of the Lyman
alpha line depends mainly on the helium/hydrogen
ratio and the anode voltage as well as on the purity of
the helium gas and the cleanliness of the vacuum sys-
tem. It is generated when impurity molecules contain-
ing hydrogen give electrons to the He + ions because of
the higher ionization potential of helium or when hy-
drogen receives energy from collisions of the second
kind with metastable helium atoms. When using mass-
spectrographic grade helium or helium evaporated
from the liquid, the Lyman alpha intensity was about
the same as the 584-A intensity in terms of numbers or
photons per second. It is for this reason that we believe
anode cooling would reduce the Lyman alpha intensity
dramatically.

There is a weak doublet of unknown origin at 1 300-A
and a few other very weak lines below 200-A. Above
2000-A the other helium emission lines and a few im-
purity lines appear. The spectrum resembles that pro-
duced by a radio-frequency discharge source. No evi-
dence of light from the He 2 + molecule ion is seen. (See
R. W. Motley and A. F. Kuckes, Proceedings of the
Fifth International Conference on Ionization Phenom-
ena in Gases, Munich, 1962 (North-Holland Publishing
Co., Amsterdam, 1962), Vol. P. 651). The source is
clean in several regions where He 2 + bands would ap-
pear, especially in the 600-900-A region and also for
the 5133-A band, and they are not observed.

Various different electrode geometries have been
investigated as well as varying numbers of electrodes.
The intensity of 584-A radiation remains roughly the
same for a given current. Since the light comes, we
believe, from bound states formed by ion-electron re-
combination, the ion densities are not appreciably al-
tered by the number or arrangement of electrodes; nor
does the ion density vary rapidly in going from cathode
to anode.

It should be realized that conditions may vary some-
what from those heretofore described when it is desired

to produce “laser” or coherent light. Experiments,
which have been performed but not yet published in
any technical paper or other news media, establish that
a device such as shown in FIG. 1 may be used to pro-
5 duce very intense beams of coherent light on a continu-
ous basis. In such an arrangement it is desirable to use
a plurality of electrodes in order to increase the current
capacity and hence the power output of the laser.
While it is true that the intensity for a given current is
10 independent of the number of electrodes, it is also true
that the magnitude of the current which can be drawn
is limited by the heat which the anode can dissipate and
that increasing the number of anodes will thus increase
the total current capacity of the device.

15 It has also been shown that this laser light may be
easily modulated in intensity at frequencies suitable for
communication and that laser action may be produced
at exceptionally short wavelengths, such as 121 6-A
hydrogen alpha, or at helium 584-A wavelengths ap-
20 propriately shifted by Doppler, Stark, isotope, or other
action so as to effectively create an inverted population
for one or more particular energies of systems.

Furthermore, it may be noted that the modification
of control features, such as by the addition of high
25 magnetic fields may be used to increase the intensity of
the light by any of the above mentioned actions. These
laser devices and applications will be described in
greater detail below in connection with FIGS. 1
through 5.

30

PLASMA DENSITY AND TEMPERATURE

Work done on the decaying plasma of the B-l stel-
larator at Princeton by Hinnov and Hirschberg has
established a convenient and reliable spectroscopic
35 method for determining electron density and tempera-
ture. This procedure was used by Robben, Kunkel, and
Talbot to determine the ion densities and temperatures
in a plasma-jet wind tunnel. This procedure depends on
electron-collision-induced transitions being dominant
40 over purely radiative transitions. When this condition is
met a kind of thermal equilibrium between electrons
and the bound states near the ionization limit is estab-
lished, providing the difference in energy between any
two neighboring bound states be small compared to kT.
45 Then the densities of these bound states, their energies
with respect to the ionization potential, and the elec-
tron temperature will be related by the Boltzmann
equation:

50 A£ nJ ./*T=ln(N„/N m ).

A E„, m is the difference in energy between states n and
m and ln(N„/N m ) is the logarithm of the ratio of the
densities of the two states.

55 The densities of these states were determined by
measuring the absolute intensities of their spectral lines
and dividing these intensities by the corresponding
transition probabilities. The transition probabilities
were calculated from the same oscillator strengths as
60 used by Robben et al. These are based on theoretical
calculations by Bates and Damgaard (See D. R. Bates
and A. Damgaard, Phil. Trans. Roy. Soc. (London)
A242, 101 (1949)), and (E. Trefftz, A. Schluter, K.
Dettmar, and K. Jorgens, Z. Astrophysc. 44, 1 (1957)
65 by both the Coulomb expansion and variational tech-
niques, which agree quite closely.

The absolute intensities of lines up to principal quan-
tum number n = 9, were measured. Unfortunately, the

4 , 007,430

7

8

spectrograph used was primarily a vacuum ultraviolet
instrument and absolute values of line intensities of
lines orginating from states higher than n = 9 did not
prove to be reliable; nor could the point where the lines
merged into the continuum be observed, which would 5
have provided a check on calculated electron densities
via the Inglis-Teller equation (See D. R. Inglis and E.
Teller, Astrophys. J. 90,439 (1939)). Fortunately, the
electron temperature was high enough so that the Bolt-
zmann relation held down to states with n = 5. The plot 10
of the logarithm of the state density divided by state
multiplicity with respect to state energy is shown in
FIG. 7 of Ser. No. 866,442. The slope corresponds to a
plasma temperature of 1660° K near the cathode. The
extrapolation of the line to E = 0 gives the logarithm of 15
(N„/g„)e _£ ” m . This is related to electron density by the
Saha equation:

N r N,lg t g, = NJg n (2nmkTlh 2 )

20

where N e , N i; and N„ are the number densities of the
electrons, ions, and bound states, respectively, and the
g’s are their multiplicities; — E„ is energy of the bound
state; g e gi(2Trmkl h 2 ) 3l2 = 1.2X10 22 if T is to be in elec-
tron volts. 25

Since the space-charge limited current of the device
is only a few milliamperes in the absence of positive
charges, the charge density of the electrons must be
essentially balanced by the charge density of the He +
ions for currents of amperes to flow. Therefore, setting 30
N e = N,- is justified and

N c 2 = N, 3 = \.2X10™1 ™(NJg„)e- ,1 »i kT .

The average ion density near the cathode is then 35
determined to be 8.4X10 12 ions/cm 3 at 30-V anode
voltage and 0.6-A anode current. The temperature
near the cathode is 1 660° K, and the average intensity
of 584-A light is 4X10 15 photon/sec/cm 3 . The ion den-
sity near the anode is 5X10 12 cm -3 at a temperature of 40
— 1900° K. The intensity of 584-A light from the anode
region is ~ 1 0 15 photons/sec/cm 3 .

In order to properly interpret the observed intensity
of 584-A radiation, it is necessary to determine the
extent to which it is resonance imprisoned. Trapped 45
radiation will be scattered out of the acceptance cone
of the spectrograph and will not be seen by it. At 300-/r
helium pressure and room temperature the mean free
path of 584-A radiation at the resonance peak is
0.0016 cm. Certainly, then much of it is trapped. 50

At these ion densities, pressures, and temperatures,
the principal broadening mechanism is the Doppler
effect due to temperature. Since the helium 2‘P atoms
which have been formed as a result of ionic recombina-
tion will be considerably hotter than the neutral helium 55
atoms, their emitted light will be Doppler shifted fur-
ther and will be expected to travel further through the
surrounding helium gas, which is at room temperature.
These excited helium atoms will usually have under-
gone only electronic collisions between the time they 60
are formed from the ions and the time they emit 584-A
radiation. They will therefore retain the temperature
associated with the ions.

The 584-A photons must travel through 2.5-cm of
helium at 300 fi to reach the differential pumping slit, at 65
which point they will not be significantly further scat-
tered in the spectrograph, and their intensity is mea-
sured. If one introduces into the light path an additional

amount of helium at 300 p. as an absorber, the 584-A
radiation will be further attenuated. The extent to
which it is attenuated will depend on the temperature
of the emitting and absorbing atoms, and on the dis-
tance which the light has already traveled through the
absorbing helium. The exact relation for the intensity
reaching a given point in the absorber, with the as-
sumption that any scattered photons are lost,

f

Y

\

Mv 2 . „ ,

1- P„\ exp

kT 1

Mv 2

-

kT

a

\

-

V

J.

/

l dv

I\ is the intensity k cm from the emitter, v the velocity
of emitting atom toward or away from observer, T^
the temperature of the emitting atom, Ta the temper-
ature of absorbing atoms, M the mass of a helium atom,
and P 0 equals 600 cm -1 , the absorption coefficient of
300° K helium at 300-fji pressure. Thus, it is improper
to speak of a mean free path when the absorber is
helium, since the absorption is not an exponential func-
tion of X.

Observations on the 5 84-A radiation from the helium
plasma give I 12 .5 / I 2.5 = 0.70. This is what one would
expect for T a —1700° K. For T cr =300° KI 12.5 / I2.5
<0. 1 9. Therefore we conclude that the emitting atoms
are at ~ 1 700° K.

At Tp. = 1700° K I 2 .5 / I 0 =0.102. The average ion
density near the cathode is ~ 10 13 /cm 3 ; however, it is
higher, ~2.5X10 13 /cm 3 , very near the cathode. At
~2.5XI0 -13 ions/cm 3 the recombination coefficient is
~2.5X10 _1 ° cm 3 /sec, taking the value of Bates et al
(See G. L. Natason, Zh. Tekhn. Fiz. 29, 1373 (1959)
English transl.: Soviet Phys-Tech. Phys., 4,1263
(I960)) which are one-half as large as those of Hinnov
and Hirschberg. If one assumed that all of the recom-
bined atoms eventually emit 584-A radiation, due to
electronic mixing between the singlet and triplet states,
one would then expect to observe.

-15X1 0 W X WI.= 1 .5X 1 0 16 photons/sec/cm 3 .

This agrees very well with the observed light intensity
and the ion densities calculated above for these re-
gions. On the basis of all of these considerations it is
concluded that ionic recombination by reaction ( 1 ) is
the mechanism of light emission.

However, a further additional point to consider is the
extent to which light produced by 2 1 P helium atoms
excited directly from the ground state would be ob-
served. This cross section for electrons (See D. R.
Bates, Atomic and Molecular Process (Acdemic Press,
N.Y., 1962, page 262)and 0. Thieme, Z. Physik. 8,412
(1932)) of energies less than 30 volt is less than
1X10 -18 cm 2 . Therefore we expect, at 0.6 amperes and
300-p.pressure, that less than 3.6X10 16 2'P excitations
would occur in one second. Since 1 2 , 5 / 1 0 for T u =300°
K is less than 0.001, one would observe less than
3.6X10 13 584-A photons per second from this mecha-
nism.

Another point is that the peak in the light intensity is
near the cathode. One would not ordinarily think that
electrons would achieve the necessary minimum of
21.5 eV so near the cathode when total anode voltage

4,007,430

9

can be as low as 22 volts. However, ions will be concen-
trated near the cathode and will recombine more rap-
idly there.

SOURCE MECHANISM

Origin of the ions: Since the space-charge-limited
current of the device described in the absence of posi-
tive charges is of the order of milliamperes, positive
ions must be present to neutralize the space charge in
order that currents as large as amperes could flow. 10
Because the source can be operated at potentials signif-
icantly below the 24.5 volt ionization potential of he-
lium, it is unlikely that ionization directly from the
ground state is involved. A two-step process involving
excitation to the 2 3 S metastable state with a maximum 15
cross section of 4x1 0~ 18 cm 2 at 20.5 eV, (See G. J.
Schultz and R. E. Fox, Phys. Rev. 106,1179 (1957))
and subsequent ionization by a second electron with a
cross section estimated to be >10~ 16 cm 2 (by compar-
ing He S 3 S to lithium (See H. Funk, Ann. Physik 20
396,149 (1930)) is possible. Another possibility is the
formation of the triplet state followed by triplet-triplet
annihilation to produce an ion, electron, and atom in
the ground state. The triplet-triplet annihilation cross
sections are very large (~10~ 14 cm 2 ) (See A. V. 25
Phelphs and J. P. Molnar, Phys. Rev. 89,1202 (1953)).

The source of Lyman alpha radiation is charge or
energy transfer from ions or metastable atoms to impu-
rities containing hydrogen. Thus, the maximum ob-
served Lyman alpha intensity of ~10 18 photons per 30
second is of interest in indicating the total possible rate
of production of ions and metastable atoms. However,
this Lyman alpha intensity was observed by injecting H 2
gas into the system. This cuased the He 584-A line to
almost disappear, so that the source mechanism may 35
have been modified somewhat.

LASER DEVICES

In FIGS. 1-4, there is shown one detailed embodi-
ment of a laser device 100 incorporating the principles 40
of this invention. The device 100 includes a plurality of
cathode wires 1 10 which are substantially of the same
thickness and material. Similarly, the device 100 in-
cludes a plurality of anode wires 111. The cathode,
wires 110 are mounted on any suitable insulators 135 45
which protrude as bosses from supporting conduits 133
and 134 so as to position the cathode wires as shown in
FIGS. 1 and 4. The conduits 133 and 134 are supported
in spaced parallel relationship to each other in the
lower portion of a vacuum chamber comprising a cylin- 50
drical container 112. The conduits and the container
may be made of either stainless steel or aluminum. In a
typical case the container 112 may be about one meter
long and approximately ten or twelve inches in diame-
ter. The ends of the container 112 are closed by circu- 55
lar end plates 130 and 131 which are hermetically
fastened to the cylinder 112 in any convenient means
as by welding or bolting to a flange, with sealing sur-
faces such as O-rings.

The conduits 133 and 134 extend through the front 60
plate 131 and are supported by it and by any suitable
attachment to the rear plate 130. The conduits 133 and
134 carry water cooling lines 136 and 137 which are
preferably arranged so as to form a separate water
cooling circuit within each conduit. The conduits 133 65
and 134 also carry the cathode filament power supply
wires 138 and 139 which are connected to a cathode
power supply and temperature control unit 121. The

10

cathode elements 110 are connected in parallel across
the cathode power lines 138 and 139. Hence, each of
the conduits 133 and 134 is required to carry only one
electrical conductor.

Similarly, a conduit 132 extends through the upper
portion of the front plate 131 and is supported thereby
and by attachment to the rear plate 130. The plurality
of anode wires 111 are mounted on conduit 132 by
insulating bosses 136. Each node wire is suspended
substantially at its midpoint by its supporting boss as
may be more clearly seen in FIG. 10. The conduit 132
also contains a water cooling circuit 135. A group of
electrical conductors 140 is also contained in conduit
132 and are connected in such a fashion that each
anode 111 is connected by a separately insulated con-
ductor to a separate and automatically regulated power
supply. These separate automatically regulated power
supplies are indicated generally by the block 122. Sepa-
rate power supplies are desirable in order to accommo-
date minor variations in operating conditions which
may occur at different points in the device.

A gas inlet line 113 also extends through the front
plate 131 of the device and may desirably continue as
to discharge near the rear plate 130. A gas outlet line

114 also extends through the front plate 131 and pref-
erably has its outlet opening positioned near the front
plate. The gas lines 113 and 114 are connected exter-
nally in closed circuit to a vacuum pump and pressure
control device 123. Of course it will be understood that
although it is preferable to continuously flow a gas
supply through the vacuum chamber of the device at a
regulated pressure in order to continuously sweep out
impurities which may be generated within the con-
tainer by operation of the device, nonetheless this fea-
ture is not essential to the operation of the device.
Although impurities may accumulate under static con-
ditions it is entirely possible for the device to operate
with gas confined at an appropriate pressure whether it
is flowing or not.

The device 100 includes a plurality of spaced elec-
trodes rather than a single pair of electrodes. This is
desirable in order to increase the total current handling
capacity of the device. It further differs in that it incor-
porates an optical system designed to produce laser
action by establishing a preferred direction for light
output by means of stimulated emission of radiation.

This optical system comprises a circular concave
mirror 115 which is mounted in an opening of corre-
sponding size in the front plate 131 so that the mirror
provides an hermetic seal for the opening. The mirror

115 is of course either a half silvered or equivalent
device having partial transmission characteristics to
provide some reflection and some light output. The
exact degree of transmission desired is a design detail
which is well understood by those skilled in the laser
art. The mirror 115 may be either planar of concave.
Furthermore, it should be noted that the output open-
ing mirror 115 may in fact be a wholely transmitting
plane window where the densities of the plasma be-
come extremely high so that multiple reflections of the
forward beam are unnecessary and small fraction of
light stimulated in the reverse direction and reflecting
off the back mirror 115' is of sufficient intensity for
stimulated emission to control the light output.

The other half of the optical system is a correspond-
ing mirror 115' mounted on the back plate 130 of the
laser 100 in any convenient mechanical fashion. Again
the mirror 115' is shown in the presently preferred

4,007,430

11

form of a concave mirror of the same concavity and
diameter as the front mirror 115. Furthermore, it will
of course be understood that the two mirrors are
aligned so that their optical axes are identical, that is to
say, so that the two mirrors are coaxially positioned 5
preferably along the central axis of the cylinder 112 so
that they produce a focusing of light through the cur-
rent flow path between the electrodes 110 and 111. As
is also well understood in the art, the distance between
these mirrors must be less than the focal length of the 10
mirrors. In a presently preferred embodiment this dis-
tance is about one meter and the mirror diameter is
about two inches.

Although the vacuum chamber or container 112 has
been shown as being formed of a single stainless steel 15
outer wall since such an arrangement gives sufficient
radiative dissipation for substantial heat loads, it will
nonetheless be understood that the entire chamber
could be water jacketed or provided with any other
suitable external cooling means should design require- 20
ments indicate this to be desirable.

In FIG. 5 there is shown a laser device Vswhich is the
same as the device 100 shown in FIG. 1, except that it
is additionally provided with an external solenoid coil
151 which is wound around the cylindrical vacuum 25
chamber 112 in such a fashion as to be coaxial with the
chamber and to extend substantially along the length
thereof in which the electrodes are positioned. The
cylinder 112 may be formed of stainless steel or of
aluminum. The solenoid 151 is connected to a current 30
supply and control device for the magnetic coil which is
identified generally as the block 152. It will of course
be understood that this device comprises a power sup-
ply with suitable conventional circuitry designed to
control the current supplied to the coil in a fashion to 35
be discussed herein. The coil is positioned and designed
so as to generate a magnetic field along the axis of the
interior of the vacuum chamber 112. The field is thus
generally at right angles to the normal electron flow of
the device 100 shown in FIG. 1. 40

The purpose of the magnetic field and the require-
ments thus placed on its current or power supply are as
follows. First, the field may be used to produce a pinch-
ing or pulsed effect in the output of the device so as to
achieve higher pulsed output levels superimposed on 45
basically continuous operation. In such use, of course,
the current supplied to the coil should be supplied in
pulses of suitable duration.

Secondly, the coil may have a continuous current of
constant amplitude supplied to it so that the magnetic 50
field is used merely to elongate the current path in the
device in order to achieve higher electron densities.
That is to say, the magnetic field can be used to force
electrons from an axially rearward cathode to flow not
to their corresponding anode as in previous embodi- 55
ments, but rather to an axially displaced anode further
forward in the device thereby increasing the current
path and the electron densities. This mode of operation
is desirable since it has been found that the light inten-
sity is proportional to a high power of the current den- 60
sity. In view of these general considerations it will of
course be understood that various field configurations
which can be produced by varying coil designs may be
desirable depending upon exactly what is wanted in any
given application. 65

With respect to both of the devices 100 and 150 of
FIGS. 1 and 5, respectively, it will be understood that
the mode of operation and the operating conditions

12

with respect to gas pressure, anode potential, cathode
power and the like are essentially the same in principle
as has been discussed in detail in connection with the
exemplary embodiment. Of course the particular de-
sign details such as choice of gases, size, power level,
etc., will be determined by the particular application
for which the device is intended. For example, if the
device is to be used as an element in a communications
system it would be necessary to provide conventional
modulating means to modulate preferably the voltage
applied to the anodes. Nonetheless, the physical source
mechanism of these laser devices of FIGS. 1 and 5 is
essentially the same as that discussed above for the
exemplary embodiment in that it is also desirable to
operate the laser device in the intermediate current
mode in order to produce a low temperature plasma
having a high density of metastable states.

Because of the extremely high densities of metastable
states attainable in these devices they make an expe-
cially powerful laser. Furthermore, laser action can be
obtained with an inverted population involving the
ground state. Operating in this mode the laser is much
more efficient than any laser constructed heretofore.

Metastable state densities of the order of 10 13 per
cubic centimeter are easily obtained with this device
operating at relatively low currents. When the device is
operated at current densities greater than one ampere
per square centimeter the densities of metastable states
is higher and approaches 10 14 per cubic centimeter.
With these devices as described having a length of
about one meter or slightly more than one yard, and
using, for example, helium with neon as the gas, it is
possible to obtain laser light from the 6328-A neon
transition of the order of 100 watts on a continuous
basis. With the device as shown in FIG. 5 modified to
provide an axial magnetic field so that one either pulses
the magnetic field to obtain extremely high densities of
emitting states during the pulse period or one uses the
magnetic field to direct the current so that it flows in a
more longitudinal manner along the axis of the laser,
considerably higher power levels are possible. These
power levels run to an order of magnitude or two larger
than the 100 watts indicated for the device of FIG. 1.

As has been mentioned before the ability to control
the density and the temperature of the plasma indepen-
dently of each other is especially beneficial. This par-
ticular feature of this device is especially noticed in
case of the helium-neon laser involving the 6328-A
neon transition. This is because the emitting neon state
has an energy very slightly greater than the helium
metastable state from which it obtains its energy. With
the ability to control the plasma temperature it is possi-
ble to control the rate at which the metastable helium
state transfers its energy to the lasing helium state and
an optimum of temperature can be chosen which will
give a maximum amount of lasing light.

Another characteristic of this apparatus is the natural
relatively large size which can be obtained. Most lasers
operating so far on the continuous basis with gases have
been small dimensionally and have been constructed
on nonconducting materials. This has made cooling
very difficult and the cooling problem has been one of
the main deterrents for these devices as far as high
power ratings are concerned. Since the device de-
scribed here is naturally encased in metal it is possible
to cool it much more efficiently and therefore to oper-
ate it at much higher power levels. Observations indi-
cate that this is a distinct advantage.

4 , 007,430

13

It has been observed that in gas lasers generally the
intensity of the laser light increases in direct proportion
to a large power of the current, the exponent haying
been observed to range from two to six. This exponen-
tial relationship means of course that one is gaining 5
laser action at an extremely significant rate when one is
operating at the very high current density levels inher-
ent in the very high power levels made possible by the
above discussed structure and heat dissipating capacity
of these devices. 10

Furthermore, in prior art lasers the two states form-
ing the inverted population have generally both been
well above the ground state in energy. It has been ex-
tremely difficult to operate a laser in which the lower
state is the ground state. This has been due to the diffi- 15
culty in obtaining an inverted population in such sys-
tems. With any significant concentration of gas the
ground state population has usually been orders of
magnitude greater than the population of the upper
state. 20

However, in the method and apparatus of the present
invention, the extremely high densities of metastable
states observed make it relatively simple to obtain laser
action in which one of the levels is the ground state.
This is a distinct advantage for two reasons. The first 25
reason is that the wavelength of the light is much
smaller than usually found which means that each pho-
ton will have a higher energy and therefore any applica-
tion in which the total energy of the photon is a consid-
eration and in which it is desirable to increase this 30
energy will benefit by laser action involving the ground
state. Secondly, since most of the energy of the system
is involved in excited atoms one can only obtain truly
efficient operation of the laser when the transition is
directly to the ground state so that one is using all of the 35
energy stored in the atoms to obtain light.

We have observed that with small amounts of hydro-
gen as the lasing gas mixed with a major portion of
helium as the buffer gas the entire hydrogen population
is inverted and exists in a high energy state. Thus, it is 40
possible to obtain laser action to the ground state from,
for example, the first excited state of the hydrogen
atom. This particular transition is of interest primarily
because it produces one of the very few types of pho- .
tons in the vacuum ultraviolet region of the electro- 45
magnetic spectrum which can be transmitted through
significant distances of ordinary air. Usually air will
absorb light in the vacuum ultraviolet. However, the
Lyman alpha light at 1 2 1 6-A wavelength which is ob-
tained from such a system is not absorbed appreciably 50
by ordinary air over distances less than a mile.

A second transition of special interest when one is
using helium as the source of metastable states in the
laser is the 584-A helium transition. Of course, it is not
possible to invert the entire helium population in the 55
device described herein. However, if one adds a small
amount of helium three, the lighter isotope of helium,
then the transition from the first excited state which
emits to the ground state, that is the state emitting the
584-A photon for the helium three, will be shifted 60
slightly in energy with respect to the helium four atoms.
This means that it is possible to obtain the helium three
in an inverted population and in this fashion to operate
the laser at very nearly 100% efficiency and to emit a
photon of such high energy that it is capable of ionizing 65
any substance except helium and neon. This same prin-
ciple may be applied to the helium-hydrogen laser sys-
tem by doping the mixture with deuterium, a heavier

14

hydrogen isotope. These are but two examples of a very
general method of efficiently moving large amounts of
radiant energy from systems producing light in the far
ultraviolet by isotopic shift techniques.

While preferred embodiments of the method and
apparatus have been described in detail above, to-
gether with specific characteristics and parameters
thereof, and while a discussion of the underlying mech-
anism of operation of the devices in accordance with
our present best understanding has been presented, it is
to be understood that this has been done only by way of
illustration and example and is not intended as a limita-
tion on the scope of the invention which is defined by
the following claims.

What is claime is:

1. The method of producing intense monochromatic
light from a continuous plasma comprising the steps of:

a. confining a gas at a controlled predetermined re-
duced pressure of at least 50 microns but below
atmospheric pressure;

b. independently controlling the temperature of an
electron emitting cathode positioned in said gas to
a constant temperature of at least 2500° K to pro-
duce an abundant supply of low-energy electrons;

c. applying a predetermined controlled low voltage of
from 20 volts to 100 volts to an anode positioned in
said gas in an electrical circuit relation with Said
cathode in which the internal resistance of the
circuit is positive so as to draw an intermediate
mode current of from 0. 1 amperes to less than 100
amperes from said cathode without arcing in order
to produce in said confined gas a region having a
high density of metastable atomic states and to thus
produce from them a low temperature, high density
plasma in said region, said intense monochromatic
light being emitted as a result of the recombination
of ions and electrons in said plasma; and

d. establishing a preferred optical direction for light
emission and output from said plasma to produce
stimulation of emission of radiation in order to
obtain laser action and thus produce coherent
light.

2. The method according to claim 1 wherein said gas
comprises a major portion of a first substance and a
minor portion of a second substance.

3. The method according to claim 2 wherein said first
substance is natural helium and said second substance
is selected from the group comprising hydrogen, neon,
argon, krypton, xenon, an isotope of these gases or an
iostope of helium.

4. The method recited in claim 2 further including
the steps of applying a magnetic field along said pre-
ferred optical direction and circulating said confined
gas through said plasma forming region in order to
remove impurities therefrom.

5. The method according to claim 1 wherein said gas
is helium confined at a pressure of about 300 microns,
said anode voltage is between 22 and 100 volts, said
current from said cathode is more than one-tenth am-
pere but less than 100 amperes, and said light has a
wavelength of 584-A.

6. The method according to claim 1 further including
the step of dissipating heat from said anode and said
cathode by placing them in heat exchange relationship
with a circulating heat transfer fluid.

7. A light source comprising:

a. means to confine a gas at a pressure above 50
microns but below atmospheric pressure;

4,007,430

15

b. output means positioned to transmit light from said
confined gas;

c. at least one anode and one cathode positioned in

said gas in electrical circuit relationship with each
other; 5

d. first control means to control the voltage applied
to said anode to between 20 volts and 100 volts,
second control means to control the temperature
of said cathode to a temperature above 2500° K by
regulating power supplied to it, and third control 10
means to control the pressure of said gas, said first,
second and third control means being arranged to
function independently of each other to permit
independent control of the temperature and den-
sity of the plasma formed in said gas when an inter- 15
mediate mode current of from 0.1 amperes to 100
amperes is drawn from said cathode to said anode;
and

e. optical means to establish a preferred light direc-
tion for stimulation of emission of radiation to 20
obtain laser action.

8. Apparatus according to claim 7 wherein said gas
comprises a major portion of a first substance and a
minor portion of a second substance and wherein said
apparatus further includes means to produce a mag- 25
netic field along said preferred light direction.

9. A laser device comprising:

a. a metallic cylindrical housing hermetically sealed
by front and rear end walls;

J 7 30

b. means to circulate a gas through said housing at a
controllable pressure reduced below atmosphere;

c. first fluid cooled supporting means to position a
plurality of anodes in longitudinally spaced rela-

16

tionship from each other in said cylindrical hous-
ing;

d. second fluid cooled supporting means to position a
corresponding plurality of cathode devices in longi-
tudinally spaced relationship from each other in
said cylindrical housing;

e. circuit means in heat conducting relation with said
second fluid cooled supporting means, said circuit
means connecting the plurality of cathodes in par-
allel circuit relationship with each other to a power
supply having means to control the temperature of
said cathodes by controlling the power supplied to
them;

f. individual circuit means in heat conducting relation
with said first fluid cooled supporting means, said
individual circuit means connecting each of said
plurality of anodes to a corresponding individual
and separately controllable voltage supply;

g. reflecting means positioned to direct light through
the plasma formed in said housing when current is
drawn from said cathode to said anode;

h. and optical output means positioned to receive
light from said reflecting means.

10. Apparatus according to claim 9 wherein a sole-
noid coil is positioned externally around said cylindri-
cal housing to generate a magnetic field along the lon-
gitudinal axis thereof; and means to control the current
supplied to said solenoid coil.

11. Apparatus according to claim 9 wherein said gas
comprises a first major portion of natural helium and a
second minor portion selected from the group compris-
ing hydrogen, neon, argon, krypton, xeonon and iso-
tope of these gases or an isotope of helium.

*****

40

45

50

55

60

65